Phenomenological model for the phase-transition sequence in betaine calcium chloride dihydrate

Abstract
Betaine calcium chloride dihydrate exhibits a sequence of over eleven structurally modulated phases, as well as a reentrant incommensurate phase. This work shows that a phenomenological model based on a one-dimensional order parameter and involving a term of the form P2(∂P/∂z)2 is adequate to account for this complex behavior.